We propose to explore the inactivation of the neuropeptide substance P in a homogenous and well defined system, the rat parotid gland. 125I-derivatives of substance P (SP) will serve as substrates. Temporal analysis of SP cleavage product in relation to disappearance of the physiological response will be carried out in order to identify the sites of cleavage of SP by the inactivation system. This information will be used in preparation of partial retro-inverso analogs of SP. This novel peptide modification reverses the direction of the amide bonds at the peptide backbone but retains the topology of the amino acids side chains at the peptide surface. It has been recently shown that partial retro-inverso modification preserves the biological specificity of leucine and methionine enkephalins and confers on the analogs remarkable metabolic stability. Systematic replacement of each amino acid residue by the corresponding N-methyl derivative will be examined as potential tool for stabilizing specific peptide bonds towards enzymatic degradation. The transient release of K ion caused by the action of SP on rat parotid slices will be used as a test system for SP analogs. The maximal amount of K ion release serves to determine the potency of the analogs while the delay in K ion reuptake is used to estimate their metabolic stability. Long-lasting analogs of SP could be extremely useful in understanding the function and potential medical applications of SP. Since previous analogs were rapidly broken down by the inactivation system the properties of these analogs could not have been previously tested.